A hypocaust (Latin hypocaustum) was an ancient Roman system of underfloor heating, used to heat houses with hot air. The word derives from the Ancient Greekhypo meaning "under" and caust-, meaning "burnt" (as in caustic). The Roman Vitruvius, writing about the end of the 1st century BCE, attributes their invention to Sergius Orata.

Hypocausts were used for heating hot baths, houses and other buildings, whether public or private. The floor was raised above the ground by pillars, called pilae stacks, with a layer of tiles, then a layer of concrete then another of tiles on top; and spaces were left inside the walls so that hot air and smoke from the furnace would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room. Ceramic box tiles were placed inside the walls to both remove the hot burned air and to heat the walls. Rooms requiring the most heat were placed closest to the furnace, whose heat could be increased by adding more wood to the fire. It was labour-intensive to run a hypocaust, as it required constant attention to tend the fire, and expensive in fuel, so it was a feature of the villa and public baths.

Caldarium from the Roman Baths at Bath, in Britain. The floor has been removed to reveal the empty spaces through which the hot air would flow.

Vitruvius describes their construction and operation in his work De architectura in about 15 BCE, adding details about how fuel could be conserved by designing the hot room or caldarium for men and women to be built next to one another, adjacent to the tepidarium so as to run the public baths efficiently. He also describes a device for adjusting the heat by a bronze ventilator in the domed ceiling.

Many remains of Roman hypocausts have survived throughout Europe, western Asia, and northern Africa. The hypocaust was an invention which improved the hygiene and living conditions of citizens, and was a forerunner of modern central heating.

Excavations at Mohenjo-daro in what is now Pakistan have unearthed what is believed to be a hypocaust lined with bitumen-coated bricks. If it fulfilled a similar role, the structure would pre-date the earliest Roman hypocaust by as much as 2000 years.

In 1984–1985, in the Republic of Georgia, excavations in the ancient settlement of Dzalisa uncovered a large castle complex, featuring a well-preserved hypocaust built between 200–400 BCE.

Dating back to 1000 BCE, Korean houses have traditionally used ondol to provide floor heating on similar principles as the hypocaust, drawing smoke from a wood fire typically used for cooking. Ondol heating was common in Korean homes until the 1960s, by which time dedicated ondol installations were typically used to warm the main room of the house, burning a variety of fuels such as coal and biomass.

With the decline of the Roman Empire, the hypocaust fell into disuse, especially in western provinces. In Britain, from c. 400 until c. 1900, central heating did not exist, and hot baths were rare.[1]

In the Iberian Peninsula, the Roman system was adopted for the heating of Hispano-Islamic (Al Andalus) baths (hammams). A derivation of hypocaust, the gloria, was in use in Castile until the arrival of modern heating. After the fuel (mainly wood) was reduced to ashes, the air intake was closed to keep hot air inside and to slow combustion.